Constructing In PQ

When it comes to new construction of the electrical infrastructure within a facility, the National Electrical Code (NEC) is the most common source of rules and regulations with regard to what to do and how to do it. Many people take the attitude that “if built to Code,” everything should work just fine, including with respect to the power quality aspects.

However, obtaining the proper quality of supply for the electrical equipment that is to be powered isn’t the point of the NEC; the purpose stated in Article 90 “is the practical safeguarding of persons and property from hazards arising from the use of electricity” and the “Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance will result in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.”

The NEC addresses some issues related to power quality phenomena, such as larger conductors for the additive triplen harmonics in neutral conductors of wye circuits. In addition, Article 647 provides “some direction for designers and installers to resolve power quality issues without creating a dangerous situation,” while Article 708 covers “the electrical installation, operation, monitoring, control, and maintenance of critical operations power systems consisting of circuits and equipment intended to supply, distribute and control electricity to designated vital operations in the event of disruption to elements of the normal system.” Relying on the NEC for constructing with power quality in mind isn’t necessarily going to get the desired results.

Used when finding the source of power quality-related problems, keeping them in mind when constructing the electrical infrastructure for a facility can minimize the potential for problems. Following these three basic rules will go a long way toward that goal:

• Lower the impedance.

• Minimize sources of noise and harmonics.

• Provide an effective grounding path.

The lower the impedance, the lower the losses from the I2 × R losses (current squared times resistance). The lower the losses, the more power reaches the intended load, and all of the things in between run cooler, since losses usually result in heat, which is the enemy of long-term life of electrical and electronic equipment. It is not just low impedance at the fundamental frequency (50 or 60 hertz) but the impedances across the whole electromagnetic spectrum, from harmonics to noise to impulsive transients. Sizing the conductor(s) properly for the type of loads may take a larger gauge than the Code requires. It may take a second conductor for the neutral in wye circuits that have significant nonlinear loads and properly rated transformers may need to be installed. Transient protection requires a low-impedance path-to-ground at the high frequencies through the use of properly rated transient-­voltage suppressors (TVSS). Even making sure all connection points are properly torqued—at busbars, breakers and even receptacles—can reduce impedance.

Without outside influences, the generator-­produced voltage would probably reach the loads as a sine wave and with voltage and current waveforms still in phase with each other. But that would mean a return to what the electrical system looked like more than 100 years ago. Voltage harmonics come from current harmonics, which come from nonlinear loads that don’t draw current continually in a full sine wave manner. Running separate circuits for “polluters,” such as large adjustable speed drives, so they are isolated from circuits that have loads susceptible to such phenomena can improve the power quality. Shielding conductors through properly grounded conduit in environments with high radio frequency and electromagnetic interference emissions can keep the voltage looking better.

Where possible, the conduit itself should not be considered the grounding conductor. A separate green wire running through the conduit provides a more effective grounding path, since all of the fittings and interconnections would have to be inspected, torqued and tightened periodically. The grounding system throughout the facility should try to establish an “equipotential” ground, where all of the points that are to be grounded have a very low impedance between them. This doesn’t mean the ground rods have to measure 1 ohm to earth, as some people think (nor does the Code require it for safety). And a single point bond for the grounded (neutral) and grounding conductors (except where there are separately derived power sources) is important.

As Article 708 points out, a key to keeping things running smoothly starts with a baseline of data during commissioning, along with ongoing tests, and is followed up with a proactive maintenance program to keep the original construction operating as closely as possible to the original state. This applies to keeping the quality of the supply compatible with the requirements of the loads as the loads and system evolve and age. NFPA 70B Recommended Practice for Electrical Equipment Maintenance, also from the National Fire Protection Association, is a good reference.